This invention relates to transistor test fixtures used in testing RF transistors in microwave frequencies (see ref. 1, 2). Active RF components (transistors—DUT) need to be thoroughly tested at the operation frequencies before used in amplifier and other circuit designs. “Load pull” and “Source pull” (see ref. 3) are test methods which use impedance tuners to systematically characterize the DUTs under various load and source impedance conditions. Load pull or source pull are automated measurement techniques used to measure Gain, Power, Efficiency and other characteristics of the DUT, employing, beyond source and load impedance tuners, also other test equipment, such as signal sources, directional couplers, test fixtures to house the DUT and input and output power meters (FIG. 1), see ref. 3. Typical test fixtures (see ref. 2) are based on micro-strip structure (FIG. 2). As can be seen in FIG. 1, the DUT is housed in the test fixture; the test fixture establishes RF connection between the DUT terminals and coaxial connectors, which allow access to and from external instruments (FIGS. 2, 4 and 5).
Microstrip test fixtures (FIG. 2A, 2B) include grounded input (22, 213) and output (28, 215) body sections, and a DUT INSERT (214). The DUT (210) is mounted on the INSERT (214) using screws (26) or other securing mechanism. The fixture has input (20) and output (27) connectors and microstrip lines (29, 212) between the connectors. The microstrip lines comprise a dielectric layer on which a thin copper conductor layer is etched using photolithographic process. The DUT (210) package has leads (24, 211, 51, 52) which can be soldered or pressed on the microstrip (29, 212), see also FIG. 5 and ref. 4.
High power transistors have very low input and output impedance ZDUT=RDUT+jXDUT, with RDUT of the order of 1 to 3Ω. For proper load pull testing, the tuners must match these impedances (i.e. create impedances RTUNER=RDUT, and XTUNER=−XDUT). However the tuning range of the tuners (i.e. the minimum RTUNER that can be reached) is reduced by the insertion loss of the test fixture; therefore there is requirement for extremely low loss test fixtures. Most existing microstrip fixtures (FIG. 2) have non-negligible insertion loss due to the used dielectric material on which the microstrip lines (29, 212) are etched photo lithographically. Since air has the lowest insertion loss of any transmission media, it is logical to make and use in a test fixture coaxial airlines, instead of micro-strip, as long as it can be technically made. In 2000/2002 such a fixture was disclosed (see ref. 1). However this fixture has 50Ω transmission airlines (31, 47, 48) which are not the optimum solution for matching low impedance targets (DUT), both from RF reflection and from mechanical points of view; as shown in FIG. 5 the transistor leads (51, 52) are wide and should also be mechanically matched to the adjoining center conductors. It is clear from FIG. 5 that the 50Ω center conductor does not match mechanically the transistor leads.